Process for the continuous digestion of cellulosic materials



May 22, 1962 A. P. SCHNYDER PROCESS FOR THE CONTINUOUS DIGESTION OF CELLULOSIC MATERIALS Filed Feb. 19, 1958 QW m6 AGENT INVENTOR fluwz'lz'msHSckny r WM 03 WZENHEDQ X050 mmoadj wzioou utzk United States Patent 3,035,963 PROCESS FOR THE CONTINUOUS DIGESTION 0F CELLULOSIC MATERIALS Auxilius P. Schnyder, Teaneck, N.J., assignor to The Lummus Company, New York, N.Y., a corporation of Delaware Filed Feb. 19, 1958, Ser. No. 716,192. 7 Claims. (Cl. 162-49) This invention relates to pulp preparation and more particularly to an improved process for effecting the continuous digestion of cellulosic materials and apparatus therefor.

At the present time the digestion of cellulosic materials is primarily carried out utilizing either soda, sulfate, sulfite or neutral sulfite processes and is primarily performed by batch methods; i.e. the cellulosic materials in relatively small pieces or chips and delignifying chemicals in aqueous solution are charged into a pressure cooker or digester wherein the mixture is raised to delignifying temperatures and thereafter discharged as concentrated pulp and spent liquor to further processing operations. Processing data for determining delignifying conditions, such as strength of the chemicals, time and temperature, is determined to a great extent from actual experience rather than from correlated laboratory data, since the severity of the chemical attack on the charged cellulosic materials varies considerably even within wood species.

The reluctance of the paper industry to convert to a continuous digestion process is for the most part based on the cost of new equipment for such process rather than on the economies of the process per se, since the new equipment for a continuously operated process would render obsolete operable batch equipment. Further, existing batch-wise operated plants have been enlarged to such an extent that certain benefits of continuous operation have been realized.

Proponents of continuous processes have long recognized that continuous process techniques when applied to the preparation of pulp, would inherently have numerous advantages over existing methods, namely:

(A) An overall continuous process from the introduction of raw cellulosic materials and delignifying chemicals to the finished product;

(B) A reduction of steam requirements;

(C) An improved pulp yield from the raw cellulosic material, including uniform pulp quality;

(D) A reduction of heat losses;

(E) Improved heat recovery systems utilized for maximum heat recovery from blown digested pulp stock;

(F) Reduced labor and operating costs; and

(G) Elimination of the time required for charging and blowing digesters.

The need for a continuous process for the digestion of cellulosic materials has resulted in many inventions, however few have attained commercial success and then only to a limited degree. For example, in U. S. Patent No. 2,200,034, a continuous process for digesting cellulosic material is described utilizing serially aligned horizontal tubular digesters. In operation, the charged cellulosic materials entering a first stage are substantially impregnated with chemical liquors at impregnation temperatures and pressures and are subsequently digested in later delignifying stages. As the delignifying reaction progresses, the cellulosic materials are passed to successive digesters by internal conveying screws mounted in each digester and thereafter are discharged from the last stage as digested pulp stock. The temperature and pressure in each digester is regulated by an individually associated 3,035,963 Patented May 22, 1962.

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recirculation and indirect heat exchange system. In U.S. Patent No. 2,474,862, a continuous process for digesting cellulosic materials is disclosed using a one stage vertical tubular digester which substantially parallels batch-operated processes, in that the charged cellulosic materials are substantially impregnated in the upper portion of the digester before attaining delignifying temperatures in the lower portion thereof. The pulp produced by this process appears to be equal to the pulp produced by batch-operated methods for similar cooking cycles.

It may be readily seen that both of the patented processes above, utilize new equipment and that the economics of constructing a plant at the expense of obsolescence of operable batch equipment becomes of paramount consideration. In order for the pulp and paper industry to accept and put into operation a continuous process for preparing pulp, economic necessity would dictate the utilization of available equipment of a prior installation unless a totally new installation is contemplated.

It is therefore a principal object of my invention to provide an improved continuously operated process for digesting cellulosic material in the preparation of pulp.

Another object of the invention is to provide an improved continuous process for digesting cellulosic material in which thorough impregnation of the material with delignifying chemicals is accomplished prior to actual digestion, thereby resulting in improved pulp products. I

A further object of the invention is the provision of an improved process for the continuous digestion of cellulosic material adaptable to existing batch-Wise operated equipment.

A still further object of the invention is to provide a continuous process for digesting cellulosic material which is easily and completely controlled with reduced labor and operating costs and which is economical with respect to steam consumption.

Other objects and a fuller understanding of my invention may be had by referring to the following description taken in conjunction with the accompanying drawing, in which the FIGURE is a flow diagram illustrating a preferred embodiment of my invention for effecting the continuous digestion of cellulosic materials in the preparation of pulp.

In accordance with my present invention, which is primarily applicable to the sulphate and soda processes and also to the long cook NSSC process, the above objects are accomplished by a novel utilization of two digesters or pressure cookers interconnected by a conduit communicating with the bottom of each digester. The cellulosic material to be digested is continuously charged to the upper zone of the first digester wherein the mate rial, while descending to the bottom thereof, is substantially impregnated with chemical liquors. The impregnated material reaching the bottom of the first digester is continuously passed through the communicating conduit to the bottom of the second digester wherein the material, while ascending to the top thereof, is substantially digested at delignifying temperatures and pressures and thereafter discharged as a mixture of concentrated pulp stock and cooking liquor from the top of the second digester. In order to continuously charge raw material to and continuously remove delignified cellulosic material from the digesters, while operating at superatmospheric pressures and high temperatures, an admission valve and a blow valve are provided at the top of the first and second digester, respectively. Such valves operate to permit the transfer of material into and out of the high pressure digesters Without an appreciable loss in the pressure medium therein. Further, a recirculation heat exchange 3 system is provided to circulate the delignifying chemicals at controlled temperatures.

In operation, I propose to impregnate the charged cellulosic materials with chemical liquors in the first digester and raise the temperature therein to about 250 F. to 325 F. and thereafter raise the impregnated materials to delignitying reaction temperatures of from about 275 F. to 370- F. in the second digester. The digested stock leaving the second digester is discharged or blown into a blow tank wherein the stock is prepared for later processing operations.

I further propose to use an outage time equal to the combined time required for substantially heating up and cooking (at maximum cooking temperatures) a batch operated process having like digester volumes, thereby determining the rate of capacity of my continuous process. The effective capacity of my process, however, may increase by 50% the capacity of existing short cooking cycle plants such as produce high yield kraft pulp and may increase by about 20% the capacity of longer cooking cycle plants, since the time required for filling, blowing and related operations of batch systems are substantially eliminated.

Referring to the drawing, there is provided a digester tank, generally indicated at 1, operating at superatmospheric pressures and at temperatures of from about 225 F. to 325 F. and functioning to receive and impregnate cellulosic materials prior to the actual digestion of such materials in a later'stage. Digester 1 is comprised of a vertical tank including an annular Wall 2, base 3 and a top 4, and associated apparatus as described hereinafter.

Within the digester tank 1, intermediate of the base and top, there is mounted on the wall 2 a peripheral recirculation strainer 5 which forms with the wall 2 an annular recirculation liquor space 5a communicating with a recirculation outlet 6. Positioned above the intersection of wall 2 and base 3, there is provided an annular distribution bafiie 7, fixed to such wall at its outer edge 7a and extending in downward configuration to a point short of the base 3, thereby forming an annular distribution space 7!) with wall 2, which space communicates with the lower portion of digester 1 through annular orifice 70 formed along edge 7d of such baffle. Recirculation liquors enter the distribution space 7b through the inlet 7e and pass through orifice 7c into the stock descending toward base 3 of the digester. Located on the exterior portion of the wall 2, there is provided a header 8 communicating with a plurality of annularly spaced distribution pipes 9 evenly disposed and mounted therein and which extend through wall 2 upwardly into the digester at'apoint above the annular distribution battle 7. Heated recirculation liquors enter the header 8 through inlet 8:: and pass through pipes 9 into the digester for mixing therein with the descending stock.

In top 4 of digester 1 there is provided an inlet pipe 10 including a gas relief outlet 11, which communicates via line 12 with a gas relief valve 13. Integral with and extending beyond the inlet pipe It there is provided an admission valve 14, which may be constructed in accord with my U.S. Patent No. 2,732,086, and which functions to continuously admit charge stock to the high pressure impregnation zone without an appreciable loss of pressure within such zone. The admission valve 14 includes a bilateral valve 15 and a'quadrant-sector valve 16 which are housed within outer casing 14a. The quadrant-sector valve 16 is mounted above the bilateral valve 15 and rotates through one-quarter revolution While the bilateral valve is motionless, whereas while the quadrant-sector valve 16 is motionless, the bilateral valve 15 rotates through one-half revolution.

The impregnated cellulosic material is primarily digested in a second digester, generally indicated at 17, operating at superatmospheric pressures and at'temperatures of from 275 F. to 370 F. Digester 17 is comprised of a vertical tank including an annular wall 18,

4 base 19 and top 20, and associated apparatus as described hereinafter.

Within the digster 17, intermediate of the base and top, there is mounted on the Wall 18, a peripheral recirculation strainer 21 which forms with the wall 18 an annular recirculation liquor space 21a communicating with a recirculation outlet 22. Axially located within the digester 17 near top 20 there is provided a directional bat-lie 23 mounted on supporting arms 23a extending from the Wall 18 and which forms therein with the wall 18 an annular space 23b for the smooth passage of concentrated stock ascending to an outlet pipe 24. Mounted on and integrally connected to pipe 24 is a blow valve 25 which may also be constructed in accord with my U.S. Patent No. 2,732,086. The blow valve 25 includes a quadrant-sector valve 26 and a bilateral valve 27 which are housed within outer casing 25a. Valve 25 operates in a manner similar to admission valve 14 in that the quadrant-sector valve 26 rotates through onequarter revolution while the bilateral valve 27 is motionless, whereas while the quadrant-sector valve 26 is motionless, the bilateral valve 27 rotates through one-half revolution.

In order to continuously transfer to digester 17 the cellulosic materials impregnated in the digester 1, there is provided 'a conduit 28 communicating with base 3 of digester 1 at one end and with the base 19 of digester 17 at the other end. The treated material descending to the bottom of digester 1 and passing into the conduit 28 is transferred to the bottom of digester 17 by high pressure steam entering the conduit 28 through a steam eductor 25 and the back pressure of recirculating liquors from digester 1'7 entering through orifice 70 as described hereinafter.

In operation, the cellulosic materials are supplied from a. chip source, such as a chip storage silo, generally indicated at 30, and may be passed via chute 31 to conveyor 32 which is provided with a weightometer 33 and a tramp iron magnet 34. The chips carried on conveyor 32 to a chip hopper 35 are continuously weighed by the weightometer 33, which controls the quantity of chemical liquor later admitted to the system. The chips from the chip hopper 35 pass through quadrant-sector valve 16 and bilateral valve 15 of admission valve 14 to digester 1, wherein the chips are impregnated with cooking liquor at superatmospheric pressures and at temperatures of from about 225 F. to 325 F. Admission valve 14 permits the cellulosic material to be continuously passed to the impregnation digester 1 without an appreciable loss in pressure therein and operates as follows; the quadrant-sector valve 16 and the bilateral valve 15 are intermittently operated, is. the quadrant-sector valve 16 rotates through one-quarter revolution while the bilateral valve is motionless, and the bilateral valve 15 rotates through one-half revolution while the quadrantsector valve is motionless.

White cooking liquor and black buffering liquor at a temperature of from F. to F. from lines 36 and 37, respectively, are passed via line 36a into the upper portion of the bilateral valve 15, wherein the liquors act 'as a snubber against steam leaking into the admission valve 14 from the digester 1. The amount of white cooking liquor and black buffering liquor entering the system is controlled by the weight of the chips passing over the Weightometer 33. Black buffering liquor from line 37 is introduced with the white liquor so as to reduce the severity of the initial attack on the cellulosic material by the white cooking liquor. The chips entering the digester tank 1 through admission valve 14 fall to the top of the chips already in the tank (as indicated by the dotted lines) together with the white cooking liquor and black buffering liquor and are subsequently impregnated with such liquors and raised to a temperature of from about 225 F. to 325 F.

As indicated, digester 1 operates at superatmospheric pressures and high temperatures, which conditions are sustained by a recirculation heat exchange system. Recirculating liquors flowing within the digester move in a direction opposite to that of the chips and leave the digester 1 through the peripheral strainer 5 and pass through zone 5a to recirculation liquor outlet 6. The liquors from outlet 6 enters line 39 and are passed by pump 40 through line 41 to heat exchanger 42. White cooking liquor and black bulfering liquor may be admitted via line 38a and 38b, respectively, into line 39 to fortify the recirculation liquors therein. Steam from steam line 43 passes via line 44 into heat exchanger 42 for indirect heat exchange with the recirculation liquors and is thereafter discharged as condensate through line 45. The quantity of steam entering heat exchanger 42 is regulated by a temperature responsive valve 44a which is controlled by a temperature probe 44b located within digester tank 1. The recirculation liquors heated by the indirect heat exchange in exchanger 42 are sent via line 46 to header 8, from which they are distributed by outlet pipes 9 into the lower portion of the digester 1. The chips descending in the digester are impregnated with the heated liquors, the countercurrent flow of which ensures for thorough impregnation. The impregnated stock descends to the portion of the digester 1 located below the bafile 7' and is passed through conduit 28 into the bottom of digester 17. The passage of the impregnated stock to the digester 17 is induced by the back pressure of recirculating liquors from the digester 17 entering digester 1 from zone 7b and passing through orifice 70 into the descending impregnated stock, and by the steam eductor 29 positioned in the conduit 28 and supplied with steam from steam supplyl43 via line 47. The quantity of steam admitted into line 28 is regulated by a temperature responsive valve 47a which is controlled by a temperature probe 47b located within digester tank 17.

Digester 17 operating at superatmospheric pressures and at temperatures of from about 275 F. to 375 F. cooks or digests the impregnated chips under delign-ifying conditions. The chemical liquor and impregnated chips in the digester 17 flow concurrently therein from bottom to top and are continuously discharged therefrom after a predetermined cooking cycle. The recirculation liquors admitted to digester 1 from distribution zone 7b to induce the passage of the impregnated stock through conduit 28 are obtained from digester 17 by withdrawing through strainer 21 a portion of the chemical liquors, and which are thereafter passed from recirculation space 21a through recirculation liquor outlet 22 into line 48. The recirculation liquors in line 48 are passed by pump 49 through line 50 and enter digester 1 through recirculation inlet 7e into the recirculation liquor zone 7b, from which the liquors are distributed into the impre nated stock near the bottom of digester 17.

The chips ascending to the top of digester 17 are substantially cooked, whereupon reaching the opening 24 are passed through blow valve 25 via line 51 into a blow tank 52 for further processing therein. Blow valve 25 comprised of quadrant-sector valve 26 and bilateral valve 27 operates to perm-it the passage of material from one pressure zone to another pressure zone without an appreciable loss of pressure therein. The operation of the blow valve 25 is similar to that of the admission valve 14, in that the quadrant-sector 26 rotates through one-quarter revolution while the bilateral valve 27 is motionless and the bilateral valve 27 rotates through one-half revolution while the quadrant-sector valve 26 is motionless.

Since the digester 17 is operating at delignifying temperatures and pressures and the blow tank is at atmospheric pressure, the materials reaching the upper portion of bilateral valve 27 are substantially blown by flash steam through line 51 into the blow tank 52.

The digested cellulosic materials entering blow tank 52 .may be treated with dilution liquors from line 53 and 54, the latter derived from the chemical liquor entering the lower portion of the bilateral valve 27 and withdrawn therefrom. The digested stock within the bottom of blow tank 52 is intimately blended and diluted by an impeller 55 to a pumpable consistency and is passed via line 56 and a pump 57 to later processing operations. The flash steam in the upper portion of the blow tank 52 rises to a flash condenser 58 associated therewith, and is condensed therein by a condensing fluid entering via line 59. The resultant condensate is passed via line 60 to a heat recovery system (not shown).

While I have shown and described a preferred form of my invention, I am aware that variations may be made thereto and I, therefore, desire a broad interpretation of my invention within the scope of the disclosure herein and the following claims.

I claim:

1. A process for continuously treating cellulosic materials at superatmospheric pressure; which comprises introducing cellulosic material to be treated into the upper portion of a first digester and passing said material as a continuous downward flow therethrough, circulating chemical treating liquor throughsaid digester in countercurrent flow to said downwardly flowing material whereby said liquor substantially impregnates said materiah.

fortifying said circulating liquor with fresh chemical treating liquor, withdrawing said impregnated material and chemical treating liquor from said digester and passing said withdrawn material and liquor to a second digester for delignification of said impregnated material during passage therethrough, heating said withdrawn material and chemical treating liquor during said passage to said second digester to about the delignifying temperature employed for said delignification, and withdrawing substantially delignified cellulosic material from said second digester.

2. A process for continuously treating cellulosic materials at superatmospheric pressure; which comprises introducing cellulosic material to be treated into the upper portion of a first digester and passing said material as a continuous downward flow therethrough, passing chemi cal treating liquor through said digester in countercurrent flow to said downwardly flowing material whereby said liquor substantially impregnates said material, separating a portion of said treating liquor from said material, heating said separated liquor to the temperature employed for said impregnation, re-introducing said heated liquorv into said digester, withdrawing said impregnated material and chemical treating liquor from said digester and passing said withdrawn material and liquor to a second digester for delignification of said impregnated material during passage upwardly therethrough, heating said withdrawn material and chemical treating liquor during said passage to said second digester to about the delignifying temperature employed for said delignification, and withdrawing substantially delignified cellulosic material from the upper portion of said second digester.

3. A process for continuously treating cellulosic materials at superatmospheric pressure; which comprises'introducing cellulosic material to be treated into the upper portion of a first digester and passing said material as a continuous downward flow therethrough, passing chemical treating liquor through said digester in countercurrent flow to said downwardly flowing material whereby said liquor substantially impregnates said material, fortifying said circulating liquor with fresh chemical treating liquor, separating chemical treating liquor from said material at a point intermediate the top and bottom of said digester, heating said separated liquor to the temperature employed for said impregnation, re-introducing said heated liquor into said digester in countercurrent flow to said downwardly flowing material, withdrawing said impregliquor substantially delignifies said material, and withdrawing substantially delignified cellulosic material from the upper portion of said second digester.

4. A process for continuously treating cellulosic materials at superatmosphen'c pressure; which comprises introducing cellulosic material to be treated into the upper portion of a first digester and passing said material as a continuous downward flow therethrough, passing chemical treating liquor through said digester in countercurrent flow to said downwardly flowing material whereby said liquor substantially impregnates said material, fortifying said circulating liquor with fresh chemical treating liquor, separating chemical treating liquor from said material at a point intermediate the top and'bottom of said digester,

heating said separated liquor to the impregnation temperature employed for said impregation, re -introducing said heated liquor into said digester in countercurrent flow to said downwardly flowing material, withdrawing said impregnated material and chemical treating liquor from said digester and passing said withdrawn material and liquor to a second digester for delignification of said impregnated material in said second digester, heating said withdrawn material and chemical treating liquor during said passage to the delignifying temperature employed for said delignification, passing said heated impregnated material and chemical treating liquor in concurrent upward flow through said second digester wherein said chemical treating liquor substantially delignifies said material, and withdrawing substantially delignified cellulosic material from the upper portion of said second digester.

5. A process for continuously treating cellulosicmaterials at superatmospheric pressure; which comprises intro .nated material and chemical treating liquor from said diducing cellulosic material to be treated into the upper portion of a first digesterand passing said material as a continuous downward flow therethrough, passing chemical treating liquor through said digester in countercilrrent flow to said downwardly flowing material whereby said liquor substantially impregnates said material, fortifying said circulating liquor with fresh chemical treating liquor, sep-, arating chemical treating liquor from said material at a point intermediate the top and bottom of said digester, heating said separated liquor to the impregnation temperature employed for said impregnation, re-introducing said heated liquor into said digester in countercurrent flow to said downwardly flowing material at a point below the point of said separation, introducing additional chemical treating liquor into said digester in concurrent flow to said downwardly flowing material, withdrawing. said impregnated material and chemical treating liquor from said digester and passing said withdrawn material and liquor to a second digester for delignification of said impregnated material during passage upwardly therethrough, heating said withdrawn material and chemical treating liquor during said passage to said second digester to about the delignifying temperature employed for said delignification, separating a portion of the chemical treating liquor from said material in said second digester at a point intermediate the top and bottom of said digester as said additional chemical treating liquor is introduced into said first digester, and withdrawing substantially delignified cellulosic material from the upper portion of said sccond digester. 6. A process for continuously treating cellulosic materials at superatmospheric pressure; which comprises introducing cellulosic material to be treated into the upper portion of a first digester and passing said material as a continuous downward flow therethrough; passing chemical treating liquor through said digester in countercurrent flow to, said downwardly flowing material whereby said liquor substantially impregnates said material, the superatrnos pheric pressure within said digester being maintained substantially constant during said impregnation; fortifying said circulating liquor with fresh chemical treating liquor,

separating chemical treating liquor from said material at a point intermediate the top and bottom of said digester;

heating said separated liquor to the impregnation temperature employed for said impregnation; re-introducing said heated liquor into said digester in countercurrent flow to said downwardly flowing material at a point below the point of said separation; introducing additional chemical treating liquor into said digester in concurrent flow to said downwardly flowing material; withdrawing said impregnated material and chemical treating liquor from said digester and passing said withdrawn material and liquor to a second digester for delignification of said impregnated material in said second digester, the superatmospheric pressure within said digester being maintained substantially constant during said 'delignification; heating said withdrawn material and chemical treating liquor during said passage to the delignifying temperature employed for said delignification; passing said heated impregnated material and chemical treating liquor in concurrent upward flow through said second digester wherein said chemical treating liquor substantially delignifies said impregnated material; separating chemical treating liquor from said material in said second digester at a point intermediate the top and bottom thereof, said separated chemical treating liquor comprising the additional chemical treating liquor admitted in concurrent flow to said material in said first digester; and withdrawing substantially delignifying cellulosic material from the upper portion of said second digester at the top thereof.

7. In a process as claimed in claim 6 wherein the heating of the continuously withdrawn material and chemical treating liquor to the delignifying temperature is accom plished by introducing steam thereto during said passage.

References Cited in the file of this patent UNITED STATES PATENTS 1,646,084 Dunbar Oct. 18, 1927 1,679,336 Dunbar July 31, 1928 1,843,466 Traquair Feb. 2, 1932 2,007,348 Scharmann et al. July 9, 1935 2,195,378 Dunbar Mar. 26, 1940 2,383,684 Ritchter Aug. 28, 1945 2,675,311 N-atwick Apr. 13, 1954 2,789,051 Obenshain Apr. 16, 1957 FOREIGN PATENTS 852,892 France Nov. 9, 1939 63,480 Norway Apr. 28, 1941 175,783 Austria Aug. 10, 1953 

1. A PROCESS FOR CONTINUOUSLY TREATING CELLULOSE MATERIALS AT SUPERATMOSPHERIC PRESSURE; WHICH COMPRISES INTRODUCING CELLULOSIC MATERIAL TO BE TREATED INTO THE UPPER PORTION OF A FIRST DIGESTER AND PASSING SAID MATERIAL AS A CONTINUOUS DOWNWARD FLOW THERETHROUGH, CIRCULATING CHEMICAL TREATING LIQUOR THROUGH SAID DIGESTER IN COUNTERCURRENT FLOW TO SAID DOWNWARDLY FLOWING MATERIAL WHERE BY SAID LIQUOR SUBSTANTIALLY IMPREGNATES SAID MATERIAL, FORTIFYING SAID CIRCULATING LIQUOR WITH FRESH CHEMICAL TREATING LIQUOR, WITHDRAWING SAID IMPREGNATED MATERIAL AND CHEMICAL TREATING LIQUOR FROM SAID DIGESTER AND PASSING SAID WITHDRAWN MATERIAL AND LIQUOR TO A SECOND DIGESTER FOR DELIGNIFICATION OF SAID IMPREGNATED MATERIAL DURING PASSAGE THERETHROUGH, HEATING SAID WITHDRAWN MATERIAL AND CHEMICAL TREATING LIQUOR DURING SAID PASSAGE TO SAID SECOND DIGESTER TO ABOUT THE DELIGNIFYING TEMPERATURE EMPLOYED FOR SAID DELIGNIFICATION, AND WITHDRAWING SUBSTANTIALLY DELIGNIFIED CELLULOSIC MATERIAL FROM SAID SECOND DIGESTER. 